When Robin Williams began his "Weapons of Self-Destruction" comedy tour in 2009, he noticed he just wasn't bouncing back after each performance as he once did. Sure, he was 58, but he still thought he felt too sluggish. He went to the doctor, who ordered a treadmill stress test to see if he had heart disease. "I was on the treadmill for five minutes and they said okay, that's enough, thank you," Williams recounted in a video his friends posted on the Internet.

Williams had aortic stenosis, a stiffening of the aortic valve, which opens to allow the heart to launch oxygen-filled blood fresh from the lungs on its journey back through the body.

In some people the valve becomes stiff and resistant to the heart's pumping action. In some cases, it's a result of childhood rheumatic fever, or a condition present since birth, or for unknown reasons. As a result, the muscle fibers in the left ventricle of the heart, just below the aortic valve, grow thicker from the exertion required to push the blood through the stiff leaflets. If the heart can no longer eject all the blood from the left ventricle, it causes, among other things, increased pressure in the lung's blood vessels, resulting in serious breathlessness.

Once serious symptoms start to appear — fainting, shortness of breath or chest pain — the average time to death is two years. The only alternative involves replacing the defective valve either with a mechanical valve made out of metal, or a valve from a pig, a cow or a human cadaver, called a homograft. About 18,000 people in the United States have their aortic valves replaced each year, according to the American Heart Association.

The surgery involves opening the chest and stopping the heart, but as radical as that may sound, aortic valve replacement is remarkably successful. Few people die from the surgery. But recovery may take six to eight weeks, which means older people — the ones most likely to need a valve replaced — often may not be vigorous enough to endure it. Even Williams found recovery arduous.

An ingenious new procedure allows surgeons to replace the aortic valve without opening the chest, which means older patients too frail for traditional surgery can now get a defective valve replaced.

"People who are 90 years old with aortic stenosis . . . there's no way they're going to have a heart operation," said Dr. William O'Neill, a professor of medicine at the University of Miami School of Medicine and the first surgeon in North America to perform the new procedure. "But they may be willing to have something less invasive like this, especially if they're very symptomatic."

The new procedure, currently limited to patients considered too old or frail for traditional aortic valve replacement, involves inserting a catheter through a small incision in the femoral artery near the groin and snaking it through the body to the aortic valve. The catheter contains a new valve, packed into an extremely tight cylinder within wire mesh. With the catheter in the valve, surgeons push the compressed valve out. It immediately pops open, squashes the diseased valve permanently against the aortic ring, and starts working.

Here's the catch: The new procedure only accommodates a valve from a pig or a cow, and those valves tend to wear out after 10 to 15 years. If you live longer than that, you may have to undergo a second valve replacement.

A mechanical valve is "incredibly durable," according to McNeill. "Those valves have remained in patients for 20, 25 years."

But mechanical valves tend to generate blood clots, which could travel straight from the aortic valve to the brain, and cause a stroke. People who receive a mechanical valve must take Coumadin, a blood thinner, for the rest of their lives, and Coumadin has the potential to thin the blood so much that it may seep out of the arteries and cause damage.

Williams, despite his relative youth, opted for a cow valve, but had traditional surgery rather than the new procedure.

That will happen less and less in the years ahead, according to O'Neill. The new procedure "will become the standard," he thinks, at least for patients in their 80s, and those who opt for a pig or cow valve. "For younger people in their 50s and 60s, the mechanical valve still has a better track record."

Of course, the mechanical valve has a big problem, as Williams told David Letterman. "If someone uses the remote control, you fart."

Tom Valeo writes frequently about health matters. He welcomes reader mail but cannot respond to individual queries. You may write him at tom.valeo@gmail.com.

Fast facts

What is aortic stenosis?

In some people the aortic valve, which opens to allow the heart to launch oxygen-filled blood fresh from the lungs on its journey back through the body, becomes stiff and resistant to the heart's pumping action. In some cases, it's a result of childhood rheumatic fever, or a condition present since birth, or for unknown reasons. As a result, the muscle fibers in the left ventricle of the heart, just below the aortic valve, grow thicker from the exertion required to push the blood through the stiff leaflets. If the heart can no longer eject all the blood from the left ventricle, it causes, among other things, increased pressure in the lung's blood vessels, resulting in serious breathlessness.

. Fast facts

What is aortic stenosis?

In some people the aortic valve, which opens to allow the heart to launch oxygen-filled blood fresh from the lungs on its journey back through the body, becomes stiff and resistant to the heart's pumping action. In some cases, it's a result of childhood rheumatic fever, or a condition present since birth, or for unknown reasons. As a result, the muscle fibers in the left ventricle of the heart, just below the aortic valve, grow thicker from the exertion required to push the blood through the stiff leaflets. If the heart can no longer eject all the blood from the left ventricle, it causes, among other things, increased pressure in the lung's blood vessels, resulting in serious breathlessness.